To create a sticky header you can add a sticky-top class while body scroll in javaScript. In bootstrap theme “sticky-top” class is used for a sticky header or element. You can add & remove your custom class with javascript. here is example of html and javascript.

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>sticky header demo</title>
  </head>
  <body>
    <header id="header_main" class="header"> ... </header>
    <h1>Hello, world!</h1>

   <script>
     window.onscroll = function() {myFunction()};

     function myFunction() {
 
     if (document.body.scrollTop > 50 ||  document.documentElement.scrollTop > 50) { document.getElementById("header_main").classList.add("sticky-top");      
} else {
   document.getElementById("header_main").classList.remove("sticky-top");      
    }    
  }  
</script>
</body>
</html>